This invention relates to a nuclear magnetic resonance (NMR) apparatus and method, especially NMR enhanced by electron spin resonance (ESR).
NMR can be used to analyze material, in the presence of an applied magnetic field and excitation pulses of r.f. energy, to ascertain the relative quantities of substances within a particular region, based on differences in the resonant frequency of different magnetic resonant (MR) active nuclei and of the same nucleus in different chemical compounds e.g. protons in fat and water (so-called spectroscopy); and NMR can be used, in the presence of an applied magnetic field and excitation pulses of r.f. energy and magnetic field gradients, to produce an image of the spatial distribution of MR active nuclei in a particular region (so-called magnetic resonance imagingxe2x80x94MRI).
The basis of NMR and MRI is the excitation of MR active nuclei to a higher energy level by r.f. pulses, and the measurement of the resulting relaxation signals created after the pulses have ceased as the nuclei return to the lower energy state. The signal-to-noise ratio, which influences image quality, depends on the difference in population of the excited and ground states. The excited state corresponds to MR active nuclei precessing about an axis opposed to the direction of the main field, and the lower energy state corresponds to the MR active nuclei precessing about an axis aligned with the direction of the main field.
Improved signal-to-noise ratio is usually produced in an MR image by increasing the strength of the applied magnetic field, since the precession frequencies of the MR active nuclei are proportional to the applied magnetic field, and the excitation pulse can have greater energy at a higher frequency. However, the magnet is one of the most expensive components of nuclear magnetic resonance apparatus, often using superconducting coils, so that higher fields are usually associated with higher costs.
This invention relates to a technique which has been developed to increase signal-to-noise ratio independent of the strength of the applied magnetic field. In this technique, which involves double resonance, atoms with unpaired electrons, usually paramagnetic, undergo electron spin resonance in the presence of an applied magnetic field together with excitation pulses of r.f. energy, this time of higher frequency than that for producing nuclear magnetic resonance (referred to hereinafter as excitation pulses of electromagnetic radiation). This resonance of the magnetic moments of the spinning electrons couples to the precessing MR active nuclei and excites them to the higher energy level. Then, when spectroscopy or imaging is carried out, higher signal-to-noise ratio in the spectroscopy or imaging is achieved.
There are few substances with unpaired electrons in the human body, so that if spectroscopy or imaging is carried out on the human body, a so-called contrast agent which does have atoms with unpaired electrons is first injected. These atoms couple with atoms of MR active nuclei and excite the latter to their higher energy level.
This so-called Overhauser enhancement has been used to image parts of the human body surrounded by coils to generate the electromagnetic radiation to produce the ESR and to generate r.f. pulses of appropriate frequency to produce NMR (GB-A-2245364, 2246201, 2252245, 2279755, 2287325 and WO 98/01766). The method is described in a publication xe2x80x9cOverhauserxe2x80x94Enhance MR Imaging (OMRI)xe2x80x9d by K Golman et. al., in Acta Radiologica 39 (1998) 10-17 (Reference 1).
Overhauser enhancement has also been used to improve the visibility of fiducial markers, by making the marker of a mixture of contrast agent and imaging agent, as described in xe2x80x9cHigh-Accuracy MR Tracking of Interventional Devices: The Overhauser Marker Enhancement (OMEN) Techniquexe2x80x9d, by Raimo P. Joensuu et. al. and published in: Magnetic Resonance in Medicine, 40:914-921 (1998) (Reference 2).
The invention provides a probe for insertion into a region of interest, which comprises a transmitter for generating an electromagnetic field in the region of interest at a frequency to excite electron spin resonance to enhance nuclear magnetic resonance in the region of interest.
The invention makes it possible to analyze localized areas.
The probe is particularly suitable for investigating the human or animal body, particularly the human body, but it could also be used for analyzing inanimate materials.
The probe is intended for use with nuclear magnetic resonance apparatus, particularly magnetic resonance imaging apparatus, although it is also suitable for use in spectroscopy. Magnetic resonance imaging apparatus comprises a magnet for producing a main magnetic field, means for producing pulses of r.f. energy, and means for applying magnetic field gradients. Magnetic resonance imaging apparatus also includes a receive coil for receiving the nuclear magnetic resonance relaxation signals, together with a transmit coil for producing the nmr r.f. excitation pulses, and the transmit coil may be the same as or distinct from the receive coil. The receive coil may be a coil which surrounds the body or a part of the body, or it may be a surface coil. However, the r.f. receive coil and/or the r.f. transmit coil, could be incorporated into the probe if desired.
The probe may include a hollow sheath with the transmitter mounted at the tip of it. An injector for injecting contrast fluid into the region of interest communicating with the hollow interior of the hollow sheath may be provided, and the sheath may form the outer conductor of a co-axial cable, the center conductor of which forms the transmitter. The center conductor may protrude from the end of the co-axial cable, forming an antenna, conveniently of one quarter wavelength in the material in which it is inserted. If desired, however, contrast fluid could be injected by means of a separate injector.
There may be provided means for generating an electromagnetic field at the transmitter for the purposes of heat treatment, for example, the use of heat to destroy diseased tissue, such as in ablation. The frequency of the transmitter for the purposes of ablation is desirably different to that from that used to excite electron spin resonance, but could if desired be the same. The nuclear magnetic resonance signal measured by the nuclear magnetic resonance apparatus may be used to control the ablation process, since the former when Overhauser enhanced is a function of the temperature of the tissue in the region of interest. The variation of the nuclear magnetic resonance signal with other factors can be compensated for by measuring the amount of electromagnetic field used to create electron spin resonance that is absorbed.
The invention also provides nuclear magnetic resonance apparatus in conjunction with the probe according to the invention.
The invention also provides apparatus for producing controlled heat treatment in a region of interest, which comprises means for generating an electromagnetic field in the region of interest at a frequency to excite electron spin resonance to enhance nuclear magnetic resonance in the region of interest, means for producing heat treatment in the region of interest, and means for using the temperature dependence of the enhanced magnetic resonance signal to control the heat treatment.